It is always good to sand first to prep the surface for better adhesion. I used 220 grit & wet sanded. Wash the pieces, let them dry & then spray a coat of sandable primer. Wetsand again using 400 grit then more primer, then a final wetsand before the finish coats. It seems like a lot of work but it goes pretty quickly. The final finish you get depends on the amount of prep you do before hand.